In gas turbine applications, coatings that have MCrAlY compositions are commonly used as oxidation resistant overlay coatings and as bond coatings for thermal barrier coating systems. The MCrAlY coating forms an aluminum oxide layer (i.e., a thermally grown oxide layer) that acts as an oxidation barrier. The composition of the MCrAlY coating is similar to many superalloys; however, there are many alloying additions made to the superalloys to improve their high temperature properties. In thermal barrier coating systems, alloying elements from the substrate diffuse through the MCrAlY coating during high temperature operations. The diffusing elements (e.g., Ta, Ti and Hf) tend to combine with the thermally grown oxide layer and reduce its effectiveness as an oxidation barrier. This results in increased oxidation rates, which leads to the failure of the thermal barrier coating system due to accelerated oxidation and growth of the thermally grown oxide layer.
Platinum modified MCrAlY coatings have been demonstrated to out-perform standard MCrAlY coatings. The platinum combines with the MCrAlY to form a Pt--Al--M phase that getters the elements that diffuse up from the substrate. The incorporation of the alloying elements from the substrate slows the degradation of the oxide layer and extends the life of the thermal barrier coating. Platinum modification improves coating life; however, platinum modified coatings are expensive, which may be prohibitive for some applications.
There is a need in the art for improved coating systems that reduce diffusion of elements from the substrate to the thermally grown oxide layer in order to increase coating life. The present invention is directed to these, as well as other, important ends.